Recently, relatively large ships, buildings, marine structures and the like have been constructed, and simultaneously, areas of application thereof have been expanded to regions of extreme cold or deep-sea regions. Since the above structures may cause catastrophic damage to the environment, as well as the loss of life and property, in a single accident, steels having high degrees of strength, extra thickness, and high low-temperature toughness have been used for such structures and vessels.
Sound, efficient welding is required along with the development of such steels, and a flux cored arc welding (FCAW) technique is a welding technique that is most widely used as a method of welding such steels.
The most widely used steels for the above structures include steels having a yield point (YP) ranging from 320 MPa to 420 MPa and a yield strength (YS) ranging from 460 MPa to 560 MPa, and weld joints of the steels must exhibit the same or better properties in comparison to base metals. In the case of being used in regions of extreme cold, the steels must also exhibit high degrees of low-temperature toughness (low-temperature impact toughness, crack tip opening displacement (CTOD)) of weld joints, while satisfying the above properties.
In general, with respect to a joint formed during welding, a portion of steel is diluted to form a melt pool, while welding materials are melted and the melt pool is subsequently solidified to become a coarse columnar structure. The structure thereof may be changed according to the welding materials and a heat input during welding. Since coarse grain boundary ferrite, Widmanstätten ferrite, martensite, and martensite austenite (M-A) constituents may be formed in coarse austenite grain boundaries of weld joints, impact toughness thereof may deteriorate significantly.
Therefore, with respect to welding materials for marine structures, refinements of welded metal structures have been pursued through the complex addition of alloying elements, such as nickel (Ni), titanium (Ti), and boron (B), along with the addition of deoxidizing, denitrifying, or dehydrogenating elements, in order to secure low-temperature Charpy impact toughness within a temperature range of about −40° C. to about −80° C. Means for improving weld toughness by the complex addition of Ti—B—Ni were developed in the early 1980s in order to meet the need for improving toughness characteristics, such as CTOD, have been commercialized, and are currently being used in various welding materials having an YP of about 550 MPa or less.
However, a mechanism for structural refinement by the complex addition of Ti-B-Ni may include matrix toughening by Ni, the inhibition of pro-eutectoid ferrite formation due to prior austenite grain boundary segregation of dissolved B, and the generation of fine ferrite in austenite grains caused by Ti, B, oxides, and nitrides.
As described above, there is a need to secure impact toughness of a weld joint by controlling the microstructure of the weld joint in order to secure stability of a welded structure. For this purpose, techniques regulating compositions of welding materials, for example, were disclosed in Japanese Patent Application Laid-Open Publication Nos. H8-10982 and H11-170085. However, since the above inventions may not control microstructures and grain diameters of welded metals, it may be difficult to secure sufficient and stable toughness of weld joints prepared with the above welding materials. Also, since changes in microstructures and compositions of weld joints may inevitably occur as heat input during welding work is changed by an amount ranging from about 0.8 kJ/mm to about 2.7 kJ/mm, it may be more difficult to secure toughness in weld joints.
Since the flux cored arc welding uses carbon dioxide gas as a protective gas during welding, it may be more economical than the case of using argon gas. However, arcs may be unstable and a decrease in welding workability, such as frequent occurrence of a spatter phenomenon in which fine particles are scattered around during welding, may occur.